The Mammalian Cell Resource (MCR) Core has two major goals and several functions in the Program Project. Its two major responsibilities are: (1) To thoroughly and carefully characterize the mammalian cell lines developed in the course of implementing the research plans within the Program Project, as well as any other cell lines that require more extensive characterization as determined by the Project Leaders, and (2) To prepare mammalian cell extracts from specific cell lines as required by the individual Projects. In addition, the MCR Core will provide the service to all Projects of maintaining cell lines as cryopreserved samples and supplying them on demand for the experimental purposes determined by the Project Leaders. These frozen samples will be derived from cell populations characterized phenotypically and certified free of mycoplasma. By centralizing and maintaining quality control of the cell lines to be used in all of the Projects, comparisons of the results of biochemical and genetic experiments across the Program Project will be facilitated, reproducibility of results will be assured, and individual investigators will be confident of inter-project comparisons. The aims of the MCR Core are: (1) To thoroughly and carefully characterize all cell lines that will be extensively used in the Program Project with respect to growth characteristics, clonogenic survival/cytotoxicity and mutation frequency responses (e.g., UVC, MMC, psoralen + UVA, IR, or others as needed), drug resistance phenotypes (e.g., 8-AA, 6-TG, HAT, ALASA, G418, etc.), and mismatch repair status/mismatch repair phenotypes, as appropriate. These characterized cell lines will then be expanded in culture and numerous cryopreserved samples from exponentially growing populations will be prepared and catalogued for distribution to the individual Projects as requested; and (2) To prepare both small (2-liter) and large (up to 50-liter) extracts from mammalian cell lines for the purposes of conducting biochemical assays (e.g., repair, incision, electrophoretic mobility shifts) as required by the individual Projects.